1. Field of the Invention
This invention relates generally to the production of multifilament yarns from synthetic polymers and more particularly to a unique process for drawing and orienting the yarns.
2. Description of the Related Art
Most fiber-forming polymers are relatively weak after they are extruded from a spinneret and cooled to form filaments. Thus, the filaments must usually be extended, or drawn, to develop optimum physical properties. This hot or cold stretching of the filaments helps to align and arrange the crystalline structure of the molecules and improves the yarn's tensile properties. A draw ratio for the multifilament yarns can be obtained by measuring the yarn's final length to original length per unit weight of yarn. During the drawing process, it is known to impinge hot air or steam on thermoplastic multifilament yarns in order to localize the draw point, reduce drawing tension, and reduce the coefficient of denier variation in the drawn products.
For example, Pitzl, U.S. Pat. No. 3,303,169 and U.S. Pat. No. 3,452,130, discloses nylon polyamide yarns and a hot fluid process for drawing them, and Pitzl, U.S. Pat. No. 3,452,132, discloses a similar process applied to polyester. Roberts et al., U.S. Pat. No. 3,048,467, concerns steam-drawing of polyolefins, particularly polyethylene and polypropylene. Bierbaum et al., U.S. Pat. No. 3,413,397, discloses steam-drawing and heat-setting of polypropylene which may be pigmented. The aforementioned references disclose other benefits of hot jet drawing, such as higher fiber tenacity, higher draw ratios, fewer broken filaments, and reduction of fiber shrinkage in boiling water. However, the benefits of hot fluid-drawing are considered economically insufficient to justify the cost on most polyamide and polyolefin production lines; as a result, hot fluid-drawing is normally used only on polyester.
A property of drawable polymeric filaments which is critically important in determining the optimum drawing conditions is the Natural Draw Ratio (NDR), described in greater detail below.
Generally, if the yarn is machine drawn at a draw ratio below the NDR of the yarn, there will be denier variation along the length of each filament, with corresponding segments of thick and thin diameter. The incompletely drawn sections of larger diameter will tend to dye darker than the more completely drawn material. The incompletely drawn sections will also appear darker if the yarns are pigmented.
Generally, if the yarn is machine drawn at a draw ratio above the NDR of the yarn, there will be less denier variation. The diameter along the length of a filament in such a yarn tends to be more uniform and there is increased fiber tenacity. Thus, it has been found that the optimum Machine Draw Ratio (MDR) for a yarn usually lies about 5-20% above its NDR. However, in some cases, it is very difficult to machine draw the yarns at such a draw ratio, because the yarns have very high NDRs. For example, the natural draw ratio for certain solution dyed nylon 66 yarns may be about 3.35:1 and possibly even greater, but the machine draw ratio for such yarns is typically set at about 2.8:1. If an attempt is made to draw such a yarn at a draw ratio 5-20% above its NDR, the process becomes inoperable with frequent filament breaks, resulting in an unacceptable product.
The present invention provides a process for drawing yarns having a natural draw ratio greater than the machine draw ratio used in the process.
Another difficult problem arises when two or more spun, but undrawn yarns, having different NDRs must be drawn on a single set of feed and draw rolls at a single machine draw ratio which is unlikely to be optimum for all of the yarns. For example, one of the yarns may have a NDR greater than the machine draw ratio and the other yarn may have a NDR less than the machine draw ratio. In other instances, both of the yarns may have different NDRs, but each is greater than the machine draw ratio.
In particular, the natural draw ratio for a yarn usually varies directly with the viscosity of the polymer and inversely with the degree of any copolymerization. Both of these factors are well known and controlled in the processing of conventional polymeric yarns. The addition of titanium dioxide as a delustrant raises the NDR, but the small amount usually employed (0.05-0.30 weight percent) has only a slight effect. Therefore, variation of the NDR has not been a serious problem in the past. More recently, larger amounts, such as 1 to 4 weight percent (wt. %), of color concentrates which include pigments and various other additives, such as UV stabilizers, antioxidants, delustrants, etc. have been incorporated into fiber-forming polymers. These color concentrates have been initially added to polyolefin polymers, since these polymers do not normally accept conventional dyes.
For example, a manufacturer of polypropylene carpet yarns has to supply its customers with a wide range of colors, some of which must be specially prepared on short notice. Pigmented polypropylene typically has a NDR of about 2.7:1 to 3.9:1. However, as shown in greater detail below, the different colorants and polymeric components of color concentrates can drastically change the NDR of the spun filaments in unpredictable directions. Thus, the optimum spinning and drawing conditions for each new color must be determined by a time-consuming and tedious process.
The drawing of solution dyed nylon carpet yarns also presents difficulties. For example, there are problems with drawing dark plum nylon yarns and light wheat nylon yarns over the same set of feed and draw rolls at a single draw ratio, because of the different natural draw ratios for the yarns. Furthermore, the dark plum nylon yarns have a higher NDR than the machine draw ratio typically used in the process.
Two references which disclose processes for drawing two or more yarns on the same set of rolls are Borenstein et al., U.S. Pat. No. 31,783, and Hatcher, U.S. Pat. No. 4,012,816, although there is no indication that the feed yarns used in these processes differ in any way.
The present invention provides a process, where two or more undrawn yarns having different natural draw ratios, at least one of which is greater than the machine draw ratio, are drawn on a single set of feed and draw rolls at a single machine draw ratio.